A spark plug is installed in a cylinder of a gasoline engine, and an air-fuel mixture that is drawn into the cylinder is ignited by a spark discharge of a spark plug.
If the concentration of the air-fuel mixture that is drawn into the cylinder is high, and the air and fuel are not sufficiently mixed, then incomplete combustion will occur, thereby producing carbon. If this carbon adheres to the outer circumference of the center electrode of the spark plug, then at the next ignition, instead of a discharge occurring between the electrodes of the spark plug, a discharge (inner flying discharge) will occur between metal attachment fittings of the spark plug and the adhering carbon. As a result, since no discharge occurs between the electrodes of the spark plug, combustion of the air-fuel mixture cannot be achieved. This condition is referred to as smoldering. In this respect, with PTL 1, when the degree of advancement of the smoldering is large, multiple discharges are generated at timings when the pressure within the combustion chamber (hereinafter, referred to as in-cylinder pressure) is higher than the in-cylinder pressure at the time of ignition. In that way, even if the spark plug is in the smoldering state, the energy at the time of discharge (energy density) can be increased. The carbon adhering to the spark plug can thereby be efficiently burned off, and a self-cleaning function of the spark plug can be enhanced.